Threat puts Dallas Police Department on high alert; no suspect or suspicious items found

Special agents from Houston are in Dallas helping with the follow-up investigation to Thursday's deadly shooting that killed five officers.

Senior Special Agent Nicole Strong works in the Bureau of Alcohol, Tobacco, Firearms and Explosives' Houston division. She arrived in Dallas shortly after she heard the news of the shooting.

"So perhaps if shell casings recovered at the scene may have been used in crimes in the past, we can help clear some of those crimes," Strong said.

Strong was inside the Dallas Police Headquarters building when officers received a threat to their safety. The SWAT team drew their weapons, officers blocked the streets and helicopters hovered overhead.

"It was very unsettling. I've been in law enforcement 25 years and just to see the tension on everyone's face, the citizens, the police, administration, visitors to other towns, everyone really appears to be on high alert," Strong said.

According to law enforcement sources, the group that made the threat to come to Dallas and kill police officers is based in Houston.

Officers searched the entire building and did not find anyone or anything suspicious.